It has been shown that T lymphocytes are primarily responsible for the cell-mediated immunity of the host. These cells are also the most important cells in the body to eliminate the neoplastic cells. The development of these cells depends upon the thymus gland. We have shown that there is a decrease of T cells in patients whose thymus glands were irradiated during early infancy. This population of patients also has a higher incidence of cancer. It is possible the decreased T lymphocytes may account for the increased incidence of cancer. Our present proposal is to extend our studies to include other populations of thymus-irradiated patients, especially the prenatal fetus, the premature infants and adult patients treated with radiation to their lung or breast. We hope these studies will determine the contributions of the thymus gland upon the T cell development during the different periods of life and the radiation effect to this gland as expressed by the proportion of T cells in the peripheral blood. BIBLIOGRAPHIC REFERENCES: Goh KO: Radiation, Cell-Mediated Immunity and Cancer, In: Handbook of Cancer and Immunology, Edited by H. Waters, Garland Publishing Inc., New York, 1977, in press. Goh KO: Chromosomes in Hematologic Diseases, In: Practical Clinical Hematology, Edited by M. Lichtman, Little, Brown & Company (Boston), 1977, in press.